Track Bed Structures

ABSTRACT

A track bed composite structural element issued in railway track engineering, and has an upper water permeable layer, a lower water permeable layer and a filter material layer provided between the upper and lower water permeable layers. The upper and lower layers may comprise geotextile layers and the filter material may be sand or glass, for example.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to track bed structures and in particular to an improved track bed structure for use in railway track engineering, ameliorating undesirable effects including sub-grade erosion.

2. State of the Art

Cohesive and fine soils have behaviour characteristics that are dominated by their fine particulate content, particularly clay and silt content. Such soils are prone to sub-grade erosion beneath a railway track bed. Mudstones and shale although appearing relatively hard compared with soils, also abrade quickly under ballast comprising the track bed, and are also prone to sub-grade erosion.

SUMMARY OF THE INVENTION

The process of sub-grade erosion begins when permeable ballast allows rainwater to percolate downwards and wet the soil below. Alternatively water may flow upwards from the soil beneath, where the sub-grade is lower than the adjacent ground water surface profile (water table). This typically occurs in cuttings or where natural springs, or artesian structures occur.

Silt structures will readily disintegrate in water, as will clay structures, but more slowly. The vibration from a train on the track above causes agitation and fluidisation and accelerates this disintegration process for silts, clay and mudrock structures. Additionally, under the effect of a passing train, pressure and deflection can cause the slurry formed by the silt and clay suspended in water to be forced by hydraulic pressure or ‘pumped’ up into the ballast. The ballast becomes contaminated with fines which eventually can result in failure of the track bed. The undesirable consequences of sub-grade erosion are ground loss from the sub-grade, loss of vertical alignment of the track and contamination of ballast leading to a detrimental effect on functional properties, such as a loss of ability for the ballast structure to hold its shape and provide the structural stability required. As a result track maintenance becomes necessary more frequently which is disruptive to transport schedules and expensive.

Previously, sub-grade erosion has been ameliorated by use of a suitable solution to the problem using blanketing sand in accordance with a UK Standard RT/CE/S/033. The sand blanket prevents rainwater from ‘ponding’ directly on the sub-grade soil. The fine sand component of the blanketing sand acts as a fine soil filter, inhibiting the passage of silts and clays. Excavation into blanketing sand has shown that the silt and clay sub-grade soil penetrates no more than a few millimetres into the blanketing sand before forming a stable soil filter. A liquid-permeable geo-textile is laid over the blanketing sand in order to prevent the ballast above penetrating into the blanketing sand. Geo-textiles are well known for use in this application and such materials which are permeable, are usually of plastics material, and commercially available from a number of manufacturers such as the material available under the trade name Terram.

Such a track bed arrangement is disclosed in FIG. 1 in which 101 represents the track bed, 102 the rails and 103 the sleepers. Standard ballast 104 lies above the geo-textile 105, which is in turn laid over the blanketing sand 106, which overlays the sub-grade material 107.

The solution described works adequately, but has severe drawbacks in terms of disruption and cost of installation.

Track renewals and repairs are frequently carried out at night and the length of track being improved has to be closed to traffic. This is a major cost. The more quickly that repairs can be effected, then the less disruption and expense.

Often, when repairing the railway track bed, the railway track is the only means of transport of materials to the repair site. This presents logistical problems.

Excavating 100 to 150 mm below ballast to make space for blanketing sand takes time. It also takes resources to have the waste material taken away. It takes time to bring in the blanketing sand unload it and spread it in an even layer of 100 mm thickness. This can be a very slow labour intensive job and, in poor working conditions at night, perhaps when it is raining or snowing and under sever time constraints. Good quality control is not always easy to achieve.

The availability of suitable grade sand is poor and can require bringing in from significant distances.

An improved technique and product have now been devised.

According to a first aspect, the present invention provides

a track bed composite structural element for use in railway track engineering, the structural element comprising an upper water permeable layer, a lower water permeable layer and a filter material layer provided between the upper and lower water permeable layers.

Typically the element comprises a panel, blanket, envelope, strip or tile like structure having opposed major faces and which in use is arranged to lie on one of its major faces. The element may be rigid, or flexible in which case it may be capable of being formed into a roll.

It is preferred that the upper, and/or the lower water permeable layers comprise geo-textile materials. Such materials are robust and suited to civil engineering situations.

Beneficially, the filter layer comprises a filter material encapsulated within the structural element.

In certain embodiments, the filter material may beneficially comprise a particulate filter material, particularly preferred grade sand filter material, such as in the grading limits specified in RT/CE/S/033. Additionally or alternatively, in certain embodiments the particulate filter material may beneficially comprise ground and/or graded glass material such as glass bead material.

It is desirable that the filter material layer is effective to filter out clay or silt, whilst permitting liquid water to pass across the element between the upper and lower water permeable layers.

It is preferred that means is provided to inhibit migration of the filter material to spaced zones of the structural element. In certain embodiments, it is preferred that the filter layer includes a dividing structure arranged to inhibit migration of the filter material to spaced zones of the structural element. The dividing structure beneficially divides the filter layer into separate zones or compartments. In certain embodiments the dividing structure may comprise a permeable structure. In certain embodiments the dividing structure may comprise an impermeable structure. The dividing structure may comprises a flexible material structure, or a rigid material structure dependent upon the embodiment.

The dividing structure divides the filter layer into a plurality of elongate compartments, such as for example elongate side by side compartments or channels.

Alternatively, the dividing structure may divide the filter layer into a grid array of cells, such a square or hexagonal cells.

Beneficially, the track bed structural element comprises a an envelope or panel having opposed major surfaces and a relatively shallow depth dimension.

In certain embodiments, it is preferred that the structural element further comprises an elongate sealing flap extending from an edge of the structural element. Beneficially, the sealing flap is secured to the lower water permeable layer of the structural element.

It certain embodiments it will be desirable that the sealing flap is water impermeable. In other situations it may be preferable that the flap is permeable. In this case the function is more protective against foreign bodies intruding into the joint rather than being liquid sealing. Preferably, the sealing flap extends along substantially an entire edge of the structural element. Also, this protects against loss of filter material from the joint through pumping or movement.

A sealing flap may also be secured to the upper water permeable layer of the structural element. The upper sealing flap may in some circumstances be permeable, however, an impermeable sealing flap is generally preferred.

According to a further aspect, the invention therefore provides a structural element for use in civil engineering applications, the structural element comprising an upper surface and a lower surface, and a filter material layer provided between the upper and lower surfaces, wherein the structural element has a sealing flap arrangement comprising an elongate sealing flap extending from an edge of the structural element.

Beneficially, the sealing flap arrangement comprises:

transversely extending marginal sealing flap portions provided along first and second transverse edges of the lower surface, transversely extending marginal sealing flap portions provided along first and second transverse edges of the upper surface, which edges are on opposite sides of the element to the edges of the flaps of the upper surface of the element.

It is preferred that the structural elements are in use laid in side by side arrangement, preferably in a in raster type order or tile pattern in which;

a first structural element is laid, a second structural element is laid to the side of the first element and on top of the side flap extending from the lower layer of the first structural element; a third structural element is laid forward of the first structural element and on top of a forward flap extending from the lower layer of the first structural element.

This is believed to be novel and inventive per se and accordingly provides a further aspect of the invention.

The laying up procedure continues in this fashion. The top flaps, where present, neatly overlay the joins along the opposed edges to the lower flaps.

It is preferred that where sealing flaps are provided on the upper layer of the structural element, that an overlaying anchor is provided upon which the ballast can be laid. The overlaying anchor can be a permeable geo-textile and/or a geo-grid layer to weight down the flaps and prevent them ‘lifting’ when the ballast is applied on top. A geo-grid adds additional strength to the arrangement.

Sealing flaps can be provided to suit particular applications and conditions in various possible permutations, specifically the element may be provided with or without the top flap arrangement, and/or with or without the bottom flap arrangement. Also the flaps may be bonded or sealed to the adjacent structural elements or not (remaining simply freely overlaying or underlaying).

-   Additionally the joints may be filled with filter material during     the laying process in order to provide filtering in the region of     the joints. This is however frequently not necessary.

According to a further aspect, the invention provides a railway track bed comprising:

a sub-grade layer; a track bed composite structural element, as herein defined; and, a ballast layer above the track bed structural element.

According to a further aspect, the invention provides a method of construction of a railway track bed, the method comprising laying a track bed composite structural element, as herein defined, on a sub-grade layer and subsequently laying a ballast layer above the track bed structural element.

The invention will flow be further described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a known track bed;

FIG. 2 is a schematic perspective view of a track bed in accordance with the invention;

FIG. 3A is a schematic perspective view of a track bed structural element in accordance with the invention;

FIG. 3B is a more detailed perspective view of the structural element of FIG. 3A;

FIG. 3C is a schematic detailed sectional view of the structural element of FIGS. 3A and 3B;

FIG. 4 is a schematic perspective view of an alternative embodiment of structural element according to the invention;

FIG. 5 is a schematic perspective view of an alternative embodiment of structural element according to the invention;

FIG. 6 is a side view of a jointed structural element arrangement according to the invention;

FIG. 7 is a detailed view of the jointed structural element arrangement of FIG. 6;

FIGS. 8A to 8C are plan and perpendicular section views respectively of an alternative embodiment of structural element according to the invention;

FIG. 9 is a schematic view of a system for laying the structural elements of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings and in particular FIG. 2, there is shown a track bed similar to that shown in FIG. 1, but in which a structural element 5 of the present invention is positioned below the ballast 4 but above the sub-grade fine soil or mudrock layer 7.

As shown in FIGS. 3A to 3C, the track bed structural element 5 comprises a an elongate structural envelope 5 having an upper liquid permeable surface 9 of geo-textile material, a lower liquid permeable surface 10, which may be identical to the upper surface or different in terms of material, thickness, permeability or other physical properties. The longitudinally opposed ends 11,12 of the structural envelope 5 are closed by end panels, which may be permeable and of geo-textile material. Longitudinally running edges 14, 15 of the structural envelope are also closed, but shown open in FIGS. 3A to 3C for explanatory purposes. A bulk filler filter material 15 is contained or encapsulated internally of the structural envelope 5. In a preferred embodiment the bulk filter material may be appropriate particulate sized filter sand. Alternative envisaged suitable materials may be other particulate bulk materials such as glass granules or beads, lightweight fibre or filter amorphous agglomerations, or mesh, net or other reticulated materials. The important physical characteristics of the bulk filler filter material 15 is that it is capable of filtering out clay silt or other sub-grade materials, especially in this particular field of use, in particle size distribution specified in accordance with, for example BS5930, whilst permitting liquid water to pass through the structural element envelope 5. The structural envelope element 5 should overall have good supporting properties in situ and preferably settle to a packed compressed configuration when in situ in the railway track bed.

In order to prevent loose particulates comprising the bulk filler filter material 15 from migrating (during transportation or laying down) from one location within the envelope 5 to another, and resulting in an unacceptably uneven distribution of the material within the envelope, the arrangement is provided with means for holding the bulk filler material together and preventing such migration to other parts of the structure. In one realisation, this is achieved by the interior of the envelope being divided into a plurality of compartments by one or more dividing structures. In the embodiment described in FIG. 3, the dividing structure comprises an internal water permeable geo-textile 18 formed serpentine to define the relevant compartments 22. In the embodiment shown, the dividing structure geo-textile 18 is a flexible geo-textile material which is bonded by means of gluing, heat bonding, pressure bonding, welding or other suitable bonding technique to the respective upper surface 9 and lower surface 10 of the envelope structure 5 along seam lines 23 running transversely to the longitudinal direction of the envelope structure 5. The dividing structure geo-textile permits liquid water to egress along or across the envelope structure without permitting the bulk filler filter material to move from compartment 22 to compartment 22. The portions of the dividing structure membrane extending between the surfaces 9 and 10 act to ‘tie’ the envelope inhibiting bulging of the structure.

The envelope structure 5 may be flexible enabling it to be stored in roll up format. Alternatively the envelope structure may be rigid, and in panel form. In panel form, like structural elements may be laid up in side by side relationship as one would lay tiles. Exemplary envisaged dimensions for a composite structural element such as that shown is depth 15 mm, 1.95 m Width (dimension X in FIG. 3A), and 0.75 m long (dimension Y in FIG. 3A). Rigid panels or roll up envelopes of such dimensions with appropriate grade sand as filler filter material should be capable of manual handling and placement. Larger structural elements 5 may require the use of mechanical handling techniques. The structural elements are laid long side 14 adjacent next panel long side 14 with the track direction being in the width direction (width Y in FIG. 3A).

FIG. 4 shows a structural element 5 generally similar in construction to the structural element of FIG. 3A (and with like structural integers denoted by like reference numerals). In the arrangement of FIG. 4, the serpentine dividing structure geo-textile 18 of FIG. 3 is replaced with a series of web elements 28 bonded to the opposed surfaces 9, 10 at margins 29. This again defines divided compartments 22.

FIG. 5 shows a structural element 5 which comprises a plastics injection moulded grid 48 having perpendicular sidewalls 41, 42 defining a series of compartment cells 44 open at their top and bottoms. The perimeter of the grid 48 is closed by a perimeter wall 49 formed integrally as part of the moulded structure of the grid. A water permeable geo-textile 59 is bonded to the upper surface of the grid 48, and a water permeable geo-textile 60 is bonded to the underside surface of the grid 48. Bulk filler filter material (such as appropriate grade sand) is present inside the compartment cells 44 and prevented from passing out of the open ends of the cells by means of the bonded geo-textile membranes. The cell walls dividing structure grid 44 permits liquid water to egress across the envelope structure 5 (bottom to top or vice versa) without permitting the bulk filler filter material to move from cell compartment 44 to cell compartment 44. The composite structural element embodiment of FIG. 4 provides a relatively rigid structural panel, but having sufficient flexibility to conform to the shape of an uneven underlaying ground surface.

Referring now to FIGS. 6 and 7, structural elements 5 and 5A are shown in side by side relationship in accordance with the invention laid on the sub-grade material 57 beneath ballast 54. Each of the structural elements 5, 5A has a water impermeable membrane flap 71 bonded to a leading longitudinal edge 14 a at the underside of the structural element 5, 5A. Bonding material bead 72 can be seen in the figure. The trailing longitudinal edge 14 b of each structural element 5 5A is provided with a preferably water impermeable membrane flap 75 bonded to the up-side of the structural element. The trailing edge 14 b of element 5 is placed adjacent the leading edge 14 a of element 5A, resting on the flap 71, and may be bonded thereto, for enhanced security of sealing, using a suitable sealant. The sealing bead 77 is shown in the figure. The top flap of 75 at the trailing edge of element 5 flaps down on the leading edge of element 5A. The flap 75 may be lifted permitting the gap between the trailing edge of element 5 and the leading edge of element 5A to be filled with sand or other suitable filler filter material, to filter out clay silt or other sub-grade materials, whilst permitting liquid water to pass. The flap 75 may be sealed down onto the leading edge of element 58 following positioning and filling of the joint (if this is desired).

Elements 5 and 5A form a close location joint. Rainwater percolating down through the ballast and entering the joint will not be able to directly enter the sub-grade soil below, because of the impermeable flap 71. the rainwater will thus be forced to enter the structural element 5 or 5A via the permeable geo-textile outer perimeter. Similarly upward flow of groundwater is forced to avoid flowing directly into the joint and must pass, at least initially into the elements 5 or 5A. The weight of the ballast and settlement caused by train passage should enhance sealing of the joint. In order to enhance sealing further the structural elements bay be overlaid initially with an anchoring structure such as a permeable geo-textile and/or other reinforcing grid, membrane or layer, before loading on of the ballast. This would aid in preventing the flaps lifting (as might otherwise occur) and ballast entering the joints, which would be undesirable. Indeed it is envisaged that in certain instances the top flaps would not be needed and for example the joints could be sand filled and a covering geo-textile or membrane overlaid.

In the embodiment of FIG. 8, the structural element 80 has a sand filled perimeter marked by the lines 81, 82, 83, 84. The underside permeable geo-textile layer overlaps the sand filled perimeter to provide an L shaped bottom marginal flap 85 (having limbs 85 a, 85 b) along the 2 perpendicular edges 81 and 84. The upper side permeable geo-textile layer overlaps the sand filled area to provide an L shaped top marginal flap 87 (having limbs 87 a, 87 b) along the 2 perpendicular edges 82 and 83.

When laying the structural elements 80 in side by side relationship a specific sequential pattern is followed. The structural elements are in use laid in side by side arrangement, preferably in a in raster type order or tile pattern (as exemplified in FIG. 9) in which a first structural element (1) is laid in the orientation as shown in FIG. 8 having the flap 85 extending to the left and forward. A second structural element (2) is laid to the side of the first element and on top of the side flap 85 b. A third element can be laid to the left of the second element (and so on, in order to build Up the required width) or, as in the sequence shown in FIG. 9, the third structural element (3) is laid forward of the first structural element (1) and on top of the forward flap 85 a of element (1). The top flaps 87 of the second and third elements overlay the firs element (1). This arrangement and laying up pattern enables the a good jointing system to be effected simply by laying up the panels in the required configuration and order. The laying up procedure continues in this fashion. The top flaps, where present, neatly overlay the joins along the opposed edges to the lower flaps.

An overlaying anchor is provided upon which the ballast can be laid. The overlaying anchor can be a permeable geo-textile or a geo-grid layer to weight down the flaps and prevent them ‘lifting’ when the ballast is applied on top.

Scaling flaps can be provided to suit particular applications and conditions in various possible permutations, specifically the element may be provided with or without the top flap arrangement, and/or with or without the bottom flap arrangement. Also the flaps may be bonded or sealed to the adjacent structural elements or not (remaining simply freely overlaying or underlaying).

Additionally the joints may be filled with sand during the laying process in order to provide filtering in the region of the joints. This is however frequently not necessary. 

1. A track bed composite structural element for use in railway track engineering, the structural element comprising: an upper water permeable layer, a lower water permeable layers and a filter material layer provided between the upper and lower water permeable layers.
 2. A track bed structural element according to claim 1, wherein the upper and lower water permeable layers comprise geo-textile materials.
 3. A track bed structural element according to claim 1, wherein the filter layer comprises a filter material encapsulated within the structural element.
 4. A track bed structural element according to claim 1, wherein: i) the filter material comprises a particulate filter material; and/or ii) the filter material comprises a sand filter material.
 5. (canceled)
 6. A track bed structural element according to claim 1, wherein the filter layer is effective to filter out clay or silt, whilst permitting liquid water to pass across the element between the upper and lower water permeable layers.
 7. A track bed structural element according to claim 1, wherein the filter layer inhibits migration of the filter material to spaced zones of the structural elements.
 8. A track bed structural element according to claim 6, wherein the filter layer includes a dividing structure arranged to inhibit migration of the filter material to spaced zones of the structural element.
 9. A track bed structural element according to claim 7, wherein: 1) the dividing structure divides the filter layer into separate zones or compartments; and/or ii) the dividing structure comprises a permeable material; and/or iii) the dividing structure comprises an impermeable material.
 10. (canceled)
 11. (canceled)
 12. A track bed structural element according to claim 7, wherein: i) the dividing structure comprises a flexible material structure; and/or ii) the dividing structure comprises a relatively rigid material structure; and/or iii) the dividing structure divides the filter layer into a plurality of elongate compartments; and/or iv) the dividing structure divides the filter layer into a grid array of cells.
 13. (canceled)
 14. (canceled)
 15. (canceled)
 16. A track bed structural element according to claim 1, in the form of an envelope or panel having opposed major surfaces and a relatively shallow depth dimension.
 17. A track bed structural element according to claim 1, further comprising an elongate sealing flap extending from an edge of the structural element.
 18. A track bed structural element according to claim 17, wherein: i) the sealing flap is secured to the lower water permeable layer; and/or ii) a sealing flap is secured to the upper water permeable layer; and/or iii) the sealing flap is water impermeable; and/or iv) the sealing flap extends along substantially an entire edge of the structural element.
 19. (canceled)
 20. (canceled)
 21. (canceled)
 22. (canceled)
 23. (canceled)
 24. (canceled)
 25. (canceled)
 26. (canceled)
 27. A railway track bed comprising: a sub-grade layer; a track bed composite structural element including an upper water permeable layer, a lower water permeable layer, and a filter material layer provided between the upper and lower water permeable layers; and a ballast layer above the track bed structural element.
 28. A railway track bed according to claim 27, wherein the structural element is laid on top of the sub-grade layer and the ballast is laid on top of the structural element.
 29. A method of construction of a railway track bed, the method comprising; laying a track bed composite structural element on a sub-grade layer and subsequently laying a ballast layer above the track bed structural element, wherein the track bed composite structural element includes an upper water permeable layer, a lower water permeable layer, and a filter material layer disposed between the upper and lower water permeable layers. 